(i) Field of the Invention
The present invention relates to a technique for determining the contact condition of electrodes in an apparatus which calculates the composition of a body by measuring a bioelectrical impedance.
(ii) Description of the Related Art
There is an apparatus which calculates fat in a body by measuring a bioelectrical impedance. As a measurement method therefor, a measurement method called a four terminal method is primarily used.
To be more specific about the four terminal method for measuring a bioelectrical impedance, the method measures an impedance in a body between parts, e.g., between both hands or both feet of a subject, and statistically estimates a body fat amount, a muscle amount, a body water amount and the like from the measurement value.
A scale-incorporating body fat meter which measures a body fat percentage as well as a body weight is known. To measure a bioelectrical impedance between both feet with the meter, a subject stands on the meter with the bottoms of both feet in contact with electrodes for feet which are provided on the top surface of the meter, and the bioelectrical impedance is then measured.
In such a measurement method, when satisfactory contact is secured between electrodes and the surface of the skin, measurement can be made without any problems, while when the contact condition of the skin surface with the electrodes is unsatisfactory, a high contact impedance occurs between the skin surface and the electrodes, and an accurate measurement may not be carried out.
As solutions for the problem, a number of methods for checking the contact condition of a subject with electrodes are proposed.
In one method, resistances Ra and Rb having values which are small enough not to influence measurement accuracy are connected to between current applying electrodes A1 and A2 and voltage measuring electrodes B1 and B2, and when a contact impedance is high, a measurement voltage which occurs in an impedance in a body can be determined by applying a high clamp voltage from a constant current circuit to a voltage measuring circuit input, thereby preventing mismeasurement. That is, the contact condition is determined by checking whether the contact impedance is a proper value (for example, Patent Document 1).
Further, there is also a method which determines whether a measurement condition is proper by determining the size of a contact impedance (for example, Patent Document 2).
Further, there is proposed a technique comprising installing a measurement start switch in electrodes to be in contact with a living body at the time of measurement so as to make it easy to obtain a satisfactory contact condition. It is described that a proper measurement posture and a proper contact condition can be obtained thereby (for example, Patent Document 3).
Further, there is proposed a method which uses current supplying electrodes as all electrodes used for measuring a bioelectrical impedance and passing a constant current I while switching connections to the electrodes so as to obtain only in-body impedance values including contact resistances and in-body impedances including contact resistance values (for example, Patent Document 4).
In addition, there is also disclosed an impedance measurement method which comprises serially connecting a plurality of reference resistances of known resistance values which can separate a bioelectrical impedance value to be measured appropriately to a living body in the route of a bioelectrical impedance measuring current, measuring voltage drop values by the resistance values of the reference resistances and a voltage drop value by the living body, determining a correlation equation between the resistance values and corresponding measured voltage values, and determining a bioelectrical impedance value by use of the measured voltage value of the living body and the correlation equation (for example, Patent Document 5).
Patent Document 1
Japanese Patent Laid-Open No. 8-154910
Patent Document 2
Japanese Patent Laid-Open No. 2003-52658
Patent Document 3
Japanese Patent Laid-Open No. 10-174680
Patent Document 4
Japanese Patent Laid-Open No. 2003-180647
Patent Document 5
Japanese Patent No. 2,835,656
For a secure measurement of a bioelectrical impedance, good contact must be established between a living body and electrodes. For this reason, as described in Description of the Related Art, methods for checking the contact condition of electrodes in measurement of a bioelectrical impedance have heretofore been proposed. These methods perform checking in addition to measurement of a bioelectrical impedance of a body part to be measured. That is, to measure a bioelectrical impedance between both feet, contact impedances between the bottoms of the feet and the electrodes are measured, and when the results are good, the contact condition of the electrodes is determined to be normal, and a bioelectrical impedance between both feet is measured. Thus, several determination processes must be performed before the measurement, resulting in a long measurement time.
Therefore, in a generally, actually used method for determining the contact condition of electrodes, whether the contact condition is satisfactory or a short may be occurring is determined by determining whether the absolute value of a measured bioelectrical impedance falls within a certain range of values.
However, there have been cases where the contact condition is misjudged as a measurable condition even when satisfactory contact is not established and measurement is actually carried out.
FIG. 9 is a block diagram of a bioelectrical impedance measuring section.
ZR represents a reference impedance corresponding to a plurality of reference resistances of known resistance values in the impedance measurement method (refer to Japanese Patent No. 2,835,656) described in Description of the Related Art, ZB represents a bioelectrical impedance to be measured, a and b represent a current supplying electrode, c and d represent a voltage measuring electrode, and ic represents a measurement current.
In general, the measurement current ic passes the reference impedance ZR and the bioelectrical impedance ZB without any problems, and complex voltages VR and VB thereof are measurable.
When a living body is not in normal contact with either or both of the current supplying electrodes a and b, the measurement circuit becomes an open state, and both VR and VB become 0, i.e., unmeasurable. At that time, a very low voltage corresponding to the common mode rejection ratio (CMRR) of an amplifier is obtained as a measurement result and is ideally 0.
Further, when either or both of the voltage measuring electrodes c and d are not in normal contact with the living body, VR can be measured, but VB becomes unmeasurable and the value thereof is not settled.
As checking in such a case, the contact condition of the electrodes is determined by setting a proper impedance range such as 200 Ω<|Z|<1000 Ω. However, in some cases, it has become stable at a value which is slightly over 200 Ω.
It is assumed that this has occurred because the voltage of a terminal with improper contact has actually occurred due to the protection circuit (e.g., diode to deal with static electricity) of the amplifier or stray capacitance (e.g., interelectrode capacitance). For example, when the voltage measuring electrode c is open (i.e., when it is not normally contacted) as shown in FIG. 9B, it is assumed that interelectrode capacitance occurs between the voltage measuring electrodes c and d and a voltage which is not supposed to be measured occurs. Thus, even when the contact condition is not normal, the conventional contact determination method using a proper impedance range misjudges that the contact condition is normal because the impedance value falls within the normal range, and continues the measurement and displays an erroneous measurement result.
The present invention has been conceived in view of such a problem. An object of the present invention is to provide an apparatus capable of carrying out an accurate bioelectrical impedance measurement by determining whether contact of a living body with electrodes used at the time of the bioelectrical impedance measurement is normal or not more accurately.